When someone tells on old-old-old joke, the recipient of the joke might reply, “The last time I heard that I fell off my dinosaur.” The line “fell off my dinosaur” has been used to describe many things that haven’t been told or that haven’t happened in a very long time. “The last time the Chicago Cubs won the World Series I fell off my dinosaur,” a long-suffering Cubs fan might say.
 
The “fell off my dinosaur” joke is old, but not ancient. “The really cool den mother reacts to that (old jokes—ed.) by saying—‘The last time I heard that I fell off my dinosaur’” was cited in print in 1967.
 
     
29 May 1967, Dallas (TX) Morning News, “Den Mother Mustn’t Lose Her Cool” by Patricia McCormack (UPI), pg. 3C, cols. 2-3:
The really cool den mother reacts to that (old jokes—ed.) by saying—“The last time I heard that I fell off my dinosaur.”
 
Google Books
September 1970, Boys’ Life “Think and Grin,” pg. 90, col. 1:
The last joke was about as funny as a submarine with screen doors. And as for the joke before that — last time 1 heard it 1 laughed so much 1 fell off my dinosaur. — Eric Russell, Mountain Lakes, N.J.
 
Google Books
Cuentos Chicanos:
A Short Story Anthology
Edited by Rudolfo A. Anaya and Antonio Márquez
Albuquerque, NM: Published for New America by the University of New Mexico Press
1984
Pg. 129:
“First time I heard that I laughed so hard I fell off my dinosaur,” Mario smiled.
 
OCLC WorldCat record
That’s so funny I fell off my dinosaur laughing
Author: Michael Pellowski
Publisher: New York : Trumpet Club, ©1988.
Edition/Format:   book_printbook : Juvenile audience : English
 
YouTube
Fell off my dinosaur
Christian Alviso
Uploaded on Feb 28, 2012
More from: SonyPictures
Paul Blart: Mall Cop 2 - Trailer 2 (Official HD)
 
Urban Dictionary
since i fell off my dinosaur
A term used to describe something that you haven’t heard or seen in forever.
Me: Dude, is that a Yugo?
Friend: Dafuq? I haven’t seen one since i fell off my dinosaur
by batmanbinsuparman June 13, 2012
 
Twitter
Kelvin Barker
‏@ChelseaKelv
The last time Spurs won at Chelsea, I was so shocked I fell off my dinosaur.
5:52 PM - 3 Dec 2014